Back to EveryPatent.com
United States Patent |
6,155,692
|
Ohkawa
|
December 5, 2000
|
Surface light source device of side light type
Abstract
A surface light source device of side light type is provided with guide
plate, a primary light source, a reflection sheet and a prism sheet. The
back surface of the guide plate provides a light control surface (arrows D
to F). This light control surface has a great number of projections
running generally at right angles to an incidence end surface. Each
projection includes a pair of slopes. Inclination angles of the slopes of
the pair with respect to a normal to a general plane of the guide plate
are equal to each other on a generally central portion of an emission
surface, while inclination angles of respective inner slopes become larger
gradually than those of respective outer slopes toward areas near both
side portions, as viewed from an incidence end surface. Length between two
side edges of the guide plate may be larger than length thereof along the
running direction of the projections, as viewed from the incidence end
surface. An emission surface of the guide plate may provide a light
control surface, in which a pair of slopes of each projection thereof may
be provided with different inclination angles in a similar fashion.
Inventors:
|
Ohkawa; Shingo (Koshigaya, JP)
|
Assignee:
|
Enplas Corporation (Kawaguchi, JP);
Yasuhiro Koike (Yakohama, JP)
|
Appl. No.:
|
168220 |
Filed:
|
October 8, 1998 |
Foreign Application Priority Data
Current U.S. Class: |
362/619; 359/628; 385/129; 385/901 |
Intern'l Class: |
F21V 007/04 |
Field of Search: |
359/628
349/65,62
385/129,146,901
362/31,26
|
References Cited
U.S. Patent Documents
5303322 | Apr., 1994 | Winston et al. | 385/901.
|
5408388 | Apr., 1995 | Kobayashi et al. | 362/31.
|
5485291 | Jan., 1996 | Qiao et al. | 385/901.
|
5659410 | Aug., 1997 | Koike et al. | 362/31.
|
5735590 | Apr., 1998 | Kashima et al. | 362/31.
|
5833344 | Nov., 1998 | Arai et al. | 362/31.
|
5838404 | Nov., 1998 | Ozeki et al. | 385/146.
|
5854872 | Dec., 1998 | Tai | 385/901.
|
5890791 | Apr., 1999 | Saito | 362/31.
|
5919551 | Jun., 1999 | Cobb, Jr. et al. | 428/156.
|
5999685 | Dec., 1999 | Goto et al. | 385/901.
|
Foreign Patent Documents |
544322 A1 | Jun., 1993 | EP | 349/65.
|
Primary Examiner: Nguyen; Thong
Assistant Examiner: Juba, Jr.; John
Attorney, Agent or Firm: Staas & Halsey LLP
Claims
What is claimed is:
1. A surface light source device of side light type, comprising:
a guide plate having two major surfaces to provide an emission surface and
a back surface; and
a primary light source to supply illumination light from an incidence end
surface of said guide plate,
wherein at least one of the two major surfaces provides a light control
surface on which a plurality of projections running generally at right
angles to the incidence end surface are arranged repetitively,
each of the projections includes a pair of slopes, and
respective inclination angles of the pair of slopes with respect to a
normal to a general plane of said guide plate are equal to each other on a
generally central portion of the emission surface, and an inner slope and
an outer slope of the pair of slopes have different angles on areas near
both side portions, as viewed from the incidence end surface.
2. The surface light source device of side light type according to claim 1,
wherein a difference between inclination angles of the pair of slopes
increases gradually from around the central portion of the emission
surface toward the areas near both side portions, as viewed from the
incidence end surface.
3. The surface light source device of side light type according to claim 1,
wherein said guide plate has a shape which is longer along the incidence
end surface than along the running direction of the projections.
4. A surface light source device of side light type, comprising:
a guide plate having two major surfaces to provide an emission surface and
a back surface; and
a primary light source for supplying illumination light from an incidence
end surface of said guide plate,
wherein the back surface provides a light control surface on which a
plurality of projections running generally at right angles to the
incidence end surface are arranged repetitively,
each of the projections includes a pair of slopes, and
respective inclination angles of the pair of slopes with respect to a
normal to a general plane of said guide plate are equal to each other on a
generally central portion of the emission surface, and an inclination
angle of an inner slope of the pair of slopes is larger than an
inclination angle of an outer slope thereof on areas near both side
portions, as viewed from the incidence end surface.
5. A surface light source device of side light type, comprising:
a guide plate having two major surfaces to provide an emission surface and
a back surface; and
a primary light source for supplying illumination light from an incidence
end surface of said guide plate,
wherein the emission surface provides a light control surface on which a
plurality of projections running generally at right angles to the
incidence end surface are arranged repetitively,
each of the projections includes a pair of slopes, and
respective inclination angles of the pair of slopes with respect to a
normal to a general plane of said guide plate are equal to each other on a
generally central portion of the emission surface, and an inclination
angle of an inner slope of the pair of slopes is smaller than an
inclination angle of an outer slope thereof on areas near both side
portions, as viewed from the incidence end surface.
6. A surface light apparatus, comprising a side light type guide plate
having an incidence end surface and two major surfaces including an
emission and back surface, wherein at least one of the major surfaces has
columns of projections arranged lengthwise at right angles to the
incidence surface and each projection includes a pair of slopes, the pair
of slopes varying based on whether the projection is in a central portion
of said guide plate or side portions of said guide plate with respect to
said incidence end surface.
7. The surface light apparatus of claim 6, wherein the columns of
projections are at least on the back surface.
8. The surface light apparatus of claim 6, wherein the columns of
projections are at least on the emission surface.
9. The surface light apparatus of claim 6, wherein the varying of slopes of
the projection is gradual from side to side of the guide plate.
10. A method of eliminating reduction in luminance around both side
portions in a surface light source,
the surface light source including a light source and a side light type
guide plate with an incidence end surface and having emission and back
surfaces, comprising:
varying slopes of projections arranged in columns lengthwise at right
angles to the incidence end surface on one of the emission or back
surfaces based on whether a projection column is on a side portion or
central portion of the guide plate with respect to said incidence end
surface.
Description
BACKGROUND
1. Field of Invention
The present invention relates to a surface light source device of side
light type, in particular to a surface light source device of side light
type in which quality of output illumination light is improved by
eliminating reduction in luminance in areas both side portion of a light
guide plate. The present invention is applicable to back lighting of a
liquid crystal display, for example.
2. Related Art
A surface light source device of side light type has conventionally been
applied to, for example, a liquid crystal display for illuminating a
liquid crystal panel from the back thereof. This arrangement is suitable
for reducing thickness over all.
The surface light source device of side light type usually employs a
rod-like light source such as a cold cathode tube as a primary light
source arranged beside a guide plate (plate-like guide body). Illumination
light emitted from the primary light source is introduced into the guide
plate through a side end surface (incidence end surface) of the guide
plate. Illumination light thus introduced propagates through the guide
plate while being emitted toward the liquid crystal panel from one
(emission surface) of major surfaces of the guide plate.
Guide plates employable for surface light source devices of side are of two
types, a type having a generally uniform thickness and another type having
thickness decreasing gradually away from the primary light source.
Generally, the latter emits illumination light more efficiently than the
former.
FIG. 6 is an exploded perspective view showing a conventional surface light
source device of side light type employing the guide plate of the latter
type. FIG. 7 shows a cross section taken in line A--A in FIG. 6. Referring
to FIGS. 6 and 7, a surface light source device of side light type 1
comprises a guide plate 2, a primary light source 3, a reflection sheet 4,
and a prism sheet 5 serving as a light control member. The reflection
sheet 4, the guide plate 3 and the prism sheet 5 are laminatedly arranged.
The primary light source 3 is arranged beside the guide plate 2.
The guide plate 2 is made of a light scattering guide body having a
wedge-shaped cross section and is called light scattering guide plate. The
light scattering guide body includes, for example, a matrix of PMMA
(polymethyl methacrylate) and a multiplicity of translucent fine particles
dispersed uniformly in the matrix. Refractive index of the fine particles
is different from that of the matrix.
The primary light source 3 includes a cold cathode tube (fluorescent lamp)
7 and a reflector 8 having a generally semicircular cross section arranged
on the back surface thereof. The reflector 8 includes a regular or
irregular reflection sheet. Illumination light is supplied toward an
incidence end surface 2A constituting a side end surface of the light
scattering guide plate 2 through an opening of the reflector 8. The
reflection sheet 4 consists of a sheet-like regular reflection member made
of a metal foil or the like or a sheet-like irregular reflection member
made of a white PET film or the like.
Illumination light is introduced into the guide plate 2 through the
incidence end surface 2A, and while being reflected between two major
surfaces (back surface 2B and emission surface 2C), propagates toward the
distal end.
In the meantime, illumination light is scattered by the fine particles in
the guide plate 2. In a case where the reflection sheet 4 made of the
irregular reflection member is employed, illumination light is also
irregularly reflected by the reflection sheet 4.
Incidence angle of illumination light to the emission surface 2C gradually
decreases according to repeated reflections by the inclined back surface
2B. Such decrease of incidence angle increases components having a
critical angle or less with respect to the emission surface and thus
promotes emission from the emission surface. As a result, lack of emission
light in an area far from the primary light source 3 is prevented.
Illumination light emitted from the emission surface 2C, which has
experienced light scattering due to the fine particles in the light
scattering guide plate 2 or irregular reflection on the reflection sheet 4
in addition thereto, has property of scattered light. The main direction
of propagation of illumination light from the emission surface 2C,
however, inclines toward a distal end with respect to the frontal
direction (in direction away from the incidence end surface 2A) regarding
in a plane row perpendicular to the incidence end surface 2A. That is, the
light emitted from the light scattering guide plate 2 has a directivity.
This property is called emission directivity.
If the back surface 2B is a smooth surface, direction of propagation of
illumination light from the emission surface 2C expands toward two sides
symmetric about the frontal direction regarding in a plane row parallel to
the incidence end surface 2A. That is, when viewed from the incidence end
surface 2A, illumination light contains components emitted obliquely
rightward and leftward. In order to correct such components directionally
and increase emission in the frontal direction, a prism surface (light
control surface) is formed also on the back surface 2B. This prism surface
has a great number of parallel prism rows. These prism rows run generally
at right angles to the incidence end surface 2A. As indicated by arrow B,
each prism row is configured of a minute projection having a triangular
cross section, for example. Slopes 2E, 2F of these projections correct
direction of light propagation so as to increase emission toward the
frontal direction regarding in a plane row parallel to the incidence end
surface 2A.
The prism sheet 5 arranged along the emission surface 2C is made of a
translucent sheet material of polycarbonate or the like. The prism sheet 5
has a prism surface formed with a great number of parallel prism rows. In
this example, the prism sheet 5 faces toward the guide plate 2 and the
prism rows are oriented so as to extend generally in parallel to the
incidence end surface 2A.
As indicated by arrow C, each prism row is configured of a minute
projection having a triangular cross section, for example. Slopes 5A, 5B
of these projections correct the obliquely-emitted illumination light in
the frontal direction regarding in a plane row perpendicular to the
incidence end surface 2A. The above-mentioned related art, however, has a
problem yet to be solved. That is, as shown in FIG. 8, in the conventional
surface light source device described above, a central area AR1 of the
emission surface provides a high luminance, whereas an areas AR2 near both
side portions provide a lower luminance level. The areas AR2 appear darker
than the area AR1. This reduced luminance tends to be more conspicuous for
a larger screen size (or a larger size of the emission surface 2C) of the
display for which the surface light source device is applied.
OBJECT AND SUMMARY OF INVENTION
The present invention solves the above-mentioned problem. An object of the
present invention is to provide a surface light source device of side
light type which is capable of effectively avoiding reduction in luminance
in areas near both side portions of the emission surface.
The present invention is applicable to a surface light source device of
side light type comprising a guide plate having two major surfaces
providing an emission surface and a back surface, and a primary light
source for supplying illumination light from an incidence end surface of
the guide plate, in which at least one of the two major surfaces of the
guide plate provides a light control surface formed with repetitive
projections running generally at right angles to the incidence end
surface. The light control surface (repetitive projections) may be formed
on either the back surface or the emission surface thereof.
According to this invention, inclination angles of a pair of slopes of each
projection of the light control surface are equal to each other at a
generally central portion of the emission surface of the guide plate, but
each inner slope (slope relatively nearer to the central portion) and each
outer slope (slope relatively farther from the central portion) have
different inclinations in areas near to both side portions as viewed from
the incidence end surface. Inclination angle of each slope is defined as
an angle with respect to a normal to a general plane of the guide plate.
It is preferable that difference of inclination angles between each pair of
slopes gradually increases toward the areas near to both side portions as
viewed from the incidence end surface. The present invention is applicable
especially advantageously to cases employing a guide plate of a shape
which is longer along the incidence end surface than along the running
direction of the projections.
More detailed features of the present invention will be understood from the
following description of embodiments with reference to the accompanying
drawings.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 exploded perspective view showing a surface light source device of
side according to a first embodiment of the present invention;
FIG. 2 is a graph or explaining directivity of illumination light emitted
from a light scattering guide plate of the surface light source device
shown in FIG. 1;
FIG. 3 is a perspective view for explaining a measurement for obtaining the
graph of FIG. 2;
FIG. 4 is a perspective view for explaining a principle of correcting
reduced luminance areas near to both side portions;
FIG. 5 in exploded perspective view showing a surface light source device
of side light type according to a second embodiment of the present
invention;
FIG. 6 is an exploded perspective view showing a conventional surface light
source device of side light type;
FIG. 7 is a sectional view taken in line A--A in FIG. 6; and
FIG. 8 is a diagram showing reduction in luminance according to prior arts.
EMBODIMENTS
(1) First Embodiment
Referring to FIG. 1, a surface light source device of side light type 10
according to a first embodiment of the present invention is illustrated to
be compared with FIG. 6. Elements common to FIG. 6 are designated by the
same reference numerals, respectively, and will not be explained in
dedistal. The surface light source device 10 comprises a primary light
source 3, a reflection sheet 4, a light scattering guide plate 12, and a
prism sheet 5 functioning as a light control member. The reflection sheet
5, the guide plate 12 and the prism sheet 5 are laminatedly arranged in
order. The primary light source 3 is arranged along a side end surface
(incidence end surface) 12A of the guide plate 12.
The guide plate 12 has a back surface 12B and an emission surface 12C as
two major surfaces. The light scattering guide plate 12 has a wedge-shaped
cross section, and the contour of the emission surface is rectangular. The
incidence end surface 12A corresponds to a long side. That is, length W
along the incidence end surface 12A is larger than length H in a direction
perpendicular to the incidence end surface 12A. In a case where a liquid
display panel having a laterally long screen is illuminated by this
surface light source device 10, the primary light source 3 is arranged, in
this illustration, along the upper side or the lower side of the liquid
crystal display panel. In other words, direction along the incidence end
surface 12A corresponds to the lateral direction as viewed by a viewer.
A material of the light scattering guide plate 12 includes a matrix of PMMA
(polymethyl methacrylate), for example, and a great number of translucent
fine particles dispersed uniformly therein. Refractive index of these fine
particles is different from that of the matrix.
The reflection sheet 4 is sheet-like regular reflection member made of a
metal foil or the like or a sheet-like irregular reflection member made of
a white PET film or the like. Illumination light L emitted from a
fluorescent lamp 7 is introduced, directly or after being reflected on a
reflector 8, into the guide plate 12 by way of the incidence end surface
12A. Illumination light thus introduced, propagates within the guide plate
12 while being repeatedly reflected between the back surface 12B and the
emission surface 12C.
The light that has leaked from the back surface 12B is reflected by the
reflection sheet 4 and returned into the guide plate 12. In the process,
the light is scattered by a great number of fine particles. If the
reflection sheet 4 is of irregular reflection type, the light is scattered
also by the reflection sheet 4. Due to the wedge-shaped cross section of
the guide plate 12, the incidence angle to the emission surface 12C
decreases at every reflection on the back surface 12C. Components
angularly below the critical angle to the emission surface 12C are emitted
from the emission surface 12C.
Projections (see arrows D, E, F) formed repeatedly on the back surface 12B
run generally at right angles to the incidence end surface 12A. These
projections correct directivity toward the frontal direction regarding in
a plane row parallel to the incidence end surface 12A. After emission from
the emission surface 12C, directivity of the light is corrected toward the
frontal direction regarding in a plane row perpendicular to the incidence
end surface 12A by slope pairs (see arrow C in FIG. 6) of a great number
of projections of the prism sheet 5. The projections of the prism sheet 5
run generally in parallel to the incidence end surface 12A.
The present embodiment is characterized by inclination of the slope pairs
of a great number of the projections formed on the back surface 12B. The
projections are repetitively formed along the incidence end surface 12A
and provide a prism surface functioning as a light control surface. Each
projection includes a pair of slopes 12E, 12F. These slopes change light
path mainly by reflection and correct directivity of illumination light
emitted from the emission surface 12C regarding in a plane row parallel to
the incidence end surface 12A. Pitch of repetition of the projections is
about 50 .mu.gm, for example. Each projection runs with a uniform
cross-sectional configuration toward a wedge distal end generally in a
direction perpendicular to the incidence end surface 12A.
What is important here is that shapes of the cross sections of projections
at a central portion (arrow D) are different from shapes of the cross
sections of projections near the side portions (arrows E, F) as viewed
from the incidence end surface 12A.
For each projection, inclination angle of one slope 12E is expressed by
.alpha., and inclination angle of the other slope 12F by .beta..
Inclination angles .alpha., .beta. are defined as angles with respect to a
normal to a general plane of the guide plate 12.
First, as plotted in partly enlarged form and designated by character D,
inclination angles .alpha. and .beta. are equal to each other at the
central portion. In this case, the slopes 12E, 12F look symmetric as
viewed from the incidence end surface 12A, and therefore, a main direction
of emission at the central portion of the emission surface 12C is
generally perpendicular to the emission surface 12C.
Near the side portions, in contrast, as plotted in partly enlarged form and
designated by characters E and F, angle .alpha. or .beta. of the inner
slope 12E or 12F is larger than angle .beta. or .alpha. of the outer slope
12F or 12E, respectively. The word "inner" means that the slope is nearer
to the center in direction along the incidence end surface 12A, and the
word "outer" means that the slope is farther from the center.
Such difference in inclination angle functions to incline the main
direction of emission toward the center of the emission surface 12C in an
area near the side portions of the emission surface 12C regarding in a
plane row parallel to the incidence end surface 12A. As a result,
reduction in luminance near the side portions (see FIG. 8) may be
effectively avoided even if the guide plate 12 has a sharp Emission
directivity.
Also, this effect has a large significance especially in a case where the
side portions correspond to the lateral direction of a laterally-long
screen. Because, if all the projections have an equally inclined slope
(.alpha.=.beta.), reduction in luminance near both side portions would be
rather striking. The above-mentioned function effectively prevents such
remarkable reduction in luminance.
It is preferable that difference between inclination angles .alpha. and
.beta. gradually decrease from the side portions toward the central
portion. This makes it possible to avoid unnatural luminance change of the
emitted light.
In a preferable example, angle .alpha. (or .beta.) of the inner slope 12E
(or 12F) is designed at 60 degrees for the right (left) side portion, and
angle .beta. (or .alpha.) of the outer slope 12F (or 12E) is designed at
10 degrees for the right (left) side portion, respectively. The central
portion is designed at .alpha.=.beta.=50 degrees.
After all, according to this embodiment, the whole emission surface 12C
appears bright when observed from the front of the central portion of the
emission surface 12C regarding in a plane row parallel to the incidence
end surface 12A (directions .+-.X in FIG. 2). In a plane row at a right
angle to the incidence end surface 12A (directions .+-.Y in FIG. 2), light
is emitted obliquely toward the wedge-shaped distal end, but this diagonal
emission may be corrected by the prism sheet 5.
Graphs (A) to (C) in FIG. 2 represent directions of emission of emitted
light as expressed by equi-brightness curves in the first embodiment.
Measuring points on three frontal positions shown in (D). As shown in FIG.
3, rightward inclination, with respect to a normal to the emission surface
12C, as viewed from the incidence end surface 12A is expressed as +X and
leftward inclination as -X. Also, inclination toward the wedge-shaped
distal end corresponds to +Y and inclination the incidence end surface
corresponds to -Y. From the graphs of FIG. 2, the following is understood.
(1) As shown in graph (B), the central portion is brightest in direction of
X=0 degree. That is, a priority emission occurs in the frontal direction
regarding in a plane parallel to the incidence end surface 12A.
(2) As shown in graph (A), the left-hand side portion and its neighborhood
leftward of the central portion as viewed from the incidence end surface
12A is brightest in direction of X=about 12 degrees. That is, a priority
emission occurs disproportionately obliquely inward from the frontal
direction regarding in a plane row parallel to the incidence end surface
12A.
(3) As shown in graph (C), the right-hand side portion and its neighborhood
rightward of the central portion as viewed from the incidence end surface
12A is brightest in direction of X=about -12 degrees. That is, a priority
emission occurs disproportionately obliquely inward from the frontal
direction regarding in a plane row parallel to the incidence end surface
12A.
These results reflect the fact that inclination of the main emission
increases toward the left and right side portions. These results are
brought about by gradually changing of difference between .alpha. and
.beta. described above (.alpha.=.beta. at the central portion).
A state in which directivity is corrected to Y=about 0 degree by the prism
sheet 5 will be studied with reference to FIG. 4. When the emission
surface 12C is observed from the frontal of the central portion of the
emission surface 12C, illumination light is emitted from each area along
the line of sight directed to the emission surface 12C.
As a result, even when the emitted light has a sharp directivity, reduction
in luminance in the side portions and the neighborhood thereof (actual
reduction in luminance for the viewer) may be effectively avoided. Also,
waste of illumination light is reduced and apparent brightness may be
increased correspondingly.
It will be obvious from the foregoing description that such an advantage
has a large significance especially in the case where the side portions of
the guide plate correspond to the lateral direction of a laterally-long
screen.
It should be noted here that such a feature results in one collateral
advantage. That is, when direction of observation is displaced slightly
from the exact front of the emission surface 12C, the display screen
darkens strikingly, thereby preventing a furtive glance by other than the
user of the display. This advantage will exhibit its effect when a
portable unit having a display with back lighting is used in a vehicle
compartment.
(2) Second Embodiment
Referring to FIG. 5, a surface light source device of side light type
according to a second embodiment of the present invention is drawn in a
manner similar to FIG. 1. Elements in this drawing shared by the
corresponding elements in FIG. 1 or 6 are designated by the same reference
numerals respectively and are described again only briefly. The surface
light source device 20 comprises the primary light source 3, the
reflection sheet 4, a light scattering guide plate 22 and the prism sheet
5 functioning as a light control member. The reflection sheet 4, the guide
plate 22 and the prism sheet 5 are laminatedly arranged. The primary light
source 3 is arranged along a side end surface (incidence end surface) 22A
of the guide plate 22.
The guide plate 22 has a back surface 22B and an emission surface 22C as
two major surfaces. The light scattering guide plate 22 has a wedge-shaped
cross section and the contour of the emission surface is rectangular. The
incidence end surface 22A corresponds to a long side. Namely, length W
along the incidence end surface 22A is larger than length H in direction
perpendicular to the incidence end surface 12A. In a case where a liquid
crystal panel having a laterally-long screen is illuminated by this
surface light source device 20, the primary light source 3 is arranged, in
the illustration, along the upper side or the lower side of the liquid
crystal display panel. In other words, direction along the incidence end
surface 22A corresponds to the lateral direction as viewed from the
viewer.
Material of the light scattering guide plate 22 is the same as that of the
light scattering guide plate 12 in the first embodiment. The reflection
sheet 4 is a sheet-like regular reflection member made of a metal foil or
the like or a sheet-like irregular reflection member made of a white PET
film or the like. Illumination light L emitted from the fluorescent lamp
7, directly or after being reflected on the reflector 8, is introduced
into the guide plate 22 from the incidence end surface 22A. Illumination
light thus introduced propagates through the guide plate 22 while being
repeatedly reflected between the back surface 22B and the emission surface
22C.
The light that has leaked from the back surface 22B is reflected on the
reflection sheet 4 and returned into the guide plate 22. In the meantime,
the light is scattered by a great number of fine particles. If the
reflection sheet 4 is of irregular reflection type, the light is also
scattered by it. Since the guide plate 22 has a wedge-shaped cross
section, the incidence angle with respect to the emission surface 22C
decreases at every reflection by the back surface 22B. Components not more
than the critical angle to the emission surface 22C are emitted from the
emission surface 22C.
A feature of the present embodiment is that the emission surface 22C, but
not the black surface 22B (compare to the first embodiment), provides a
light control surface (prism surface). This light control surface has a
great number of projections.
The projections (see arrows D, E, F) formed repetitively on the emission
surface 22C run generally at right angles to the incidence end surface
22A. These projections correct directivity in the frontal direction
regarding in a plane row parallel to the incidence end surface 22A. After
emission from the emission surface 22C, directivity is corrected toward
the frontal direction regarding in a plane row perpendicular to the
incidence end surface 22A by slope pairs of a great number of the
projections of the prism sheet 5 (see arrow C in FIG. 6). The projections
of the prism sheet 5 run generally parallel to the incidence end surface
22A.
This embodiment is characterized by inclination of the slope pairs of a
great number of the projections formed on the emission surface 22C. Each
projection has a pair of slopes 22E and 22F. These slopes change light
path mainly by refraction to correct directivity of illumination light
emitted from the emission surface 22C regarding in a plane row parallel to
the incidence end surface 22A. Repetition pitch of the projections is
about 50 .mu.gm, for example. Each projection runs with a uniform cross
section toward the distal of the wedge in direction generally at right
angles to the incidence end surface 22A.
What is important is that the shape of the cross section (arrow D) of the
projections on the central portion is different from that of the
projections on the side portions (arrows E, F) as viewed from the
incidence end surface 22A.
Like in FIG. 1, inclination angle of one slope 22E of each projection is
expressed by a and inclination angle of the other slope as .beta..
Inclination angles .alpha., .beta. are defined as angles with respect to a
normal to a general plane of the guide plate 22.
On the central portion (character D), inclination angles .alpha. and .beta.
are equal to each other. In this case, the slopes 22E, 22F are symmetric
about the incidence end surface 22A, and therefore the main direction of
emission on the central portion of the emission surface 22C is a direction
generally normal to the emission surface 22C.
On the side portions, in contrast, as drawn in partly enlarged form and
designated by reference characters E and F, angle .alpha. or .beta. of the
inner slope 22E or 22F of each projection is smaller than angle .beta. or
.alpha. of the outer slope 22F or 22E, respectively. The word "inner"
means that the slope is nearer to the center in direction along the
incidence end surface 22A, and the word "outer" means that the slope is
far from the center.
Such difference of inclination angle functions to incline the main
direction of emission toward the center of the emission surface 22C in an
area near the side portions of the emission surface 22C regarding in a
plane row parallel to the incidence end surface 22A. As a result,
reduction in luminance (see FIG. 8) near the side portions may be
effectively avoided even if the guide plate 22 has a sharp emission
directivity.
Also, this effect has a large significance especially in a case where the
side portions correspond to the lateral direction of a laterally-long
screen. Because, if all the projections have equal slopes (.alpha.=.beta.)
in such a case, reduction in luminance near both side portions would be
rather striking. The function described above effectively avoids such a
remarkable reduction in luminance.
preferably difference between inclination angles .alpha. and .beta.
gradually decreases from the side portions toward the central portion. As
a result, unnatural change in luminance of the emitted light may be
avoided.
According to one preferable example, angle .alpha. (or .beta.) of the inner
slope 22E (or 22F) is designed at 10 degrees for the right (left) side
portion, and angle .beta. (or .alpha.) of the outer slope 22F (or 22E) is
designed at 60 degrees for the right (left) side portion. The central
portion is designed at .alpha.=.beta.=50 degrees
It will be apparent from the foregoing description that the present
embodiment also has operations and advantages similar to those of the
first embodiment. That is, as described above with reference to FIG. 4,
when the emission surface 22C is viewed from the frontal direction of the
central portion of the emission surface 22C, illumination light is emitted
from each area along the line of sight directed to the emission surface
22C.
As a result, even if the emitted light has a sharp directivity, reduction
in luminance near the side portions (actual reduction in luminance for a
viewer) may be effectively avoided. Also, waste of illumination light may
be correspondingly reduced and apparent brightness may be increased.
It will be apparent that these advantages have a large significance
especially in the case where the side portions of the guide plate
correspond to the lateral direction of a laterally-long screen. A furtive
glance from an adjacent seat may be prevented in a similar manner to the
first embodiment.
(3) Modifications
The embodiments described above are not intended to limit the scope of the
present invention. For example, the following modifications are possible.
(i) Pitches of the projections formed on the back surface or the emission
surface of the guide plate may not be constant. For example, pitches may
be varied partially. Also, the height of the projections may not be
constant.
(ii) In the embodiments described above, the projections on the light
control surface form a triangular cross section by direct connection of a
pair of slopes. However, such a configuration does not limit the present
invention. For example, a pair of slopes may be connected by a smooth
curved surface. Also, the slope itself may be formed as a curved surface.
(iii) In the embodiments described above, projections having a uniform
cross-sectional configuration are formed from the incidence end surface
toward the wedge distal. This, however, does not limit the invention. For
example, the shape of the cross-sectional configuration of the projections
may be varied from the incidence end surface toward the wedge distal.
(iv) The emission surface of the guide plate may be, wholly or partly,
provided with the light scattering function by matting process, applying
ink or others.
(v) The present invention is applicable also to a case which employs a
guide plate having both the back surface and the emission surface thereof
formed with projections.
(vi) The translucent material constituting the prism sheet is not
specifically limited. For example, polymethyl methacrylate (PMMA) or the
like may be employed. Also, the prism sheet preferably has a flexibility.
Nevertheless, an element having substantially no flexibility such as what
is called a prism body may also be employed.
(vii) The prism sheet may be eliminated in some cases. Also, the present
invention is applicable to a case in which a light scattering sheet for
scattering illumination light is arranged in place of or in addition to
the prism sheet.
(viii) In the first embodiment described above, a light- scattering guide
plate containing translucent fine particles was employed. However, other
types of light scattering guide plate may be used. Also, a transparent
guide plate may be employed.
(ix) The cross-sectional configuration of the guide plate is not
necessarily wedge-shaped. For example, a guide plate having a uniform
thickness may be employed.
(x) The incidence surface of the guide plate may be set on two or more end
surfaces. A plurality of primary light sources may be provided
correspondingly.
(xi) The primary light source may include a light source element other than
a rod-like light source such as a fluorescent lamp. For example, a
plurality of pointlike light sources such as light emitting diodes may be
arranged to form the primary light source.
(xii) Surface light source devices according to this invention may be
applied to other than to back lighting of a liquid crystal display. For
example, it is widely applicable to various illumination equipments and
displays.
Top